Many words that existed in Old English did not survive into Modern English. There are also many words in Modern English that bear little or no resemblance in meaning to their Old English etymons. Some linguists estimate that as much as 80 percent of the lexicon of Old English was lost by the end of the Middle English period, including many compound words, e.g. bōchūs ('bookhouse', 'library'), yet the components 'book' and 'house' were kept. Certain categories of words seem to have been more susceptible. Nearly all words relating to sexual intercourse and sexual organs as well as "impolite" words for bodily functions were ignored in favor of words borrowed from Latin or Ancient Greek. The Old English synonyms are now mostly either extinct or considered crude or vulgar, such as arse/ass. Some words were forgotten while other near-synonyms in Old English replaced them ('limb' remains in common use, but lið remains only dialectally as lith). Many of these changes came with the introduction of Old Norse and Norman French words, while others fell away due to natural evolution. Animals Modern English has no Germanic words for 'animal' in the general sense of 'non-human being'. Old English dēor, gesceaft, gesceap, nēat and iht were all eclipsed by 'animal', 'beast', 'creature' and 'critter'. ācweorna: squirrel. Displaced by Anglo-Norman esquirel and Old French escurel, from Vulgar Latin scuriolus, diminutive of scurius, variant of Latin sciurus, from Ancient Greek σκίουρος (skíouros). Compare German Eichhorn, Dutch eekhoorn, Icelandic ikorni, Swedish ekorre, Danish egern, Norwegian ekorn. āðexe: 'lizard'. Lizard was borrowed into Middle English from Old French lesarde, from Latin lacertus. The earliest occurrence of the word (spelled lusarde) is in the poem Piers Plowman (written about 1360–1399). Old English āðexe does survive dialectally as ask ('newt', 'eft', 'lizard'): cf. German Eidechse, Dutch hagedis. ælepūte: 'burbot'. The Old French word borbote had replaced ælepūte by the Middle English period. 'Burbot' first occurred in English around 1475. Its modern descendant, 'eelpout', is occasionally used for the burbot, although it has come to define a different animal. cāwelwyrm: 'caterpillar' (see lēafwyrm). culfre: 'dove', 'pigeon' has survived dialectally as 'culver', which the AHD believes comes from Vulgar Latin colombula. The OED acknowledges this possibility, but asserts that it is more likely native. 'Culver' is first attested in English around 825 and 'dove' around 1200. The Middle English dove is thought to come from Old English, but the assumed